1. Field of the Invention
This invention relates to the art of manufacturing flat glass wherein molten glass is delivered onto a surface of a pool of molten metal and formed while floating on the molten metal into a continuous sheet of glass. More particularly, this invention relates to improved barriers positioned in the glass-supporting molten metal to control convection currents or flow of molten metal within the pool.
2. Brief Description of the Prior Art
The use of dam barriers to influence thermal conditions within a glass-supporting pool of molten metal in a glass-forming chamber has long been recognized. For example, U.S. Pat. No. 789,911 to Hitchcock discloses the use of a plurality of barriers to segregate a glass-supporting pool of molten metal into a plurality of pool segments, each of which may be selectively maintained at a desired temperature to permit the cooling and forming of glass as it passes over each segment of the pool during its travel through a forming chamber.
U.S. Pat. No. 3,317,302 to Misson discloses the use of dam barriers which are sufficiently beneath the glass-supporting surface of a pool of molten metal so that, while segregating the pool into several segments, they permit the flow of molten metal along the length of the forming chamber to a desired degree to effect the cooling of the glass during its formation.
U.S. Pat. No. 3,584,475 to Galey and Sensi discloses dam barriers extending transversely across the bottom of a glass-forming chamber which are contoured to provide for a greater inhibition of molten metal flow along the margins of a forming chamber than in the central portion of the forming chamber.
U.S. Pat. No. 3,930,829 to Sensi discloses the use of dam barriers in a forming chamber, some of which are positioned inwardly from the side walls of the forming chamber so that the flow of molten metal along the extreme marginal portions of the forming chamber is not inhibited by the presence of a dam barrier while the flow of molten metal throughout a major central portion of the forming chamber is inhibited by the presence of the dam barriers. In this patent, Sensi also discloses the use of short dam barriers which do not extend across the full width of a forming chamber such as dam barriers 280 shown in FIG. 14 of U.S. Pat. No. 3,930,829.
While the above inventions have lead to improved glass, they have not overcome the problem of longitudinal defects caused by uneven thickness of the glass that results because of uneven temperatures transversely across the glass during the ribbon forming of the float formation process. The methods of the prior art all require symmetrical barriers, dams or bottom pieces that produce either a single return and advance flow of molten metal or else matching flows of molten metal forward on each side of the top and backward along each side of the float bath. These flows may be either between adjacent dam barriers or along the surface and bottom of the entire tank of molten metal. These various currents all contribute to the longitudinal defects of the glass as the movement of the molten metal with the symmetrical dams and barriers of the prior art tends to produce a temperature effect at the middle of the molten metal bath. Therefore, there is a need for a float glass formation process which will eliminate the difficulty of a temperature gradient across the transverse direction of the glass as it moves through the float forming chamber and particularly the concentration of temperature difference at the middle of the sheet as it is formed producing the centerline and other longitudinal defects which are a thickness variation caused by the uneven cooling which the glass receives across the transverse direction from the tin or other molten metal.